The Treasury issued temporary regulations yesterday governing one of the more controversial sections of President Reagan's tax package--buying and selling corporate tax breaks through transactions called leases--but postponed a formal ruling on one of the key issues.

At a briefing with reporters, John B. Chapoton, assistant secretary for tax policy, defended provisions that are expected to double or triple the arcane leasing business as "an integral part" of the accelerated capital depreciation system in the tax act, which took effect Oct. 1.

He said they are essential to prevent stepped-up corporate takeovers based entirely on tax considerations and to give companies with little or no profits a chance to take advantage of the new tax breaks.

However, the department reserved a decision on the legality of what has become known in the trade as an investment tax credit strip lease, in which a company would sell to another company the 10 percent investment credit.

The effect of the reservation, along with language that private lawyers said indicates strip leases will be very difficult to consummate, will be to prevent the practice altogether before a Nov. 13 deadline the companies face. By that time, firms must convert into leases any investment credits they have dating back to the first of the year.

Although Chapoton declined to characterize the regulations as either liberal or restrictive, lease brokers and lawyers said some elements were more restrictive than they had expected.

They suggested that the tight rules will function both to keep revenue losses down and to make lease transactions with marginal companies such as Chrysler Corp. and International Harvester Co. less inviting to profitable companies. "These regulations are not going to help the companies that are really in trouble," one lawyer commented.

Approval of strip leases would have permitted a profitable company to have effectively purchased an investment tax credit from a company unable to use the credit because its profits were too low. The depreciation tax breaks would have remained with the low-profit company using the equipment.

Such a mechanism was attractive because it would have been "quick and dirty," in the words of one lawyer involved in the leasing business, a sale of a tax break that could be accomplished within a short period of time with little risk to the company buying the tax credit through a lease.

The 1981 tax bill broadly expanded the definition of leases. Under the new law, leases can be transactions that exist almost entirely on paper in which corporate tax benefits are bought and sold.

A company running in the red, or close to it, such as an airline or automobile maker, can buy new equipment, sell it for tax purposes only to a profitable company, such as an oil firm or computer maker, and then lease it back. The profitable company then takes advantage of the tax breaks, which were useless to the automobile maker or airline, and the net cost of the purchase of the equiptment is reduced.

In addition to the failure to rule on these strip leases, another section of the rules is expected to inhibit leasing with marginal companies, particularly those that face the possiblity of bankruptcy.